Resistance welding is now widely used in most applications requiring the joining of metals, such as the steel used in the manufacturing of automobiles. With the advent of the microprocessor, weld controllers have become more sophisticated and use a variety of control techniques to ensure the quality of welds throughout the life of the contact tips as they wear out. Regardless of the process or control technique used, most weld controllers consist of several basic components. These include a weld control module, a power module, a weld transformer and the contact tips. The power module usually consists of power semiconductors such as silicon controlled rectifiers (SCRs) that switch incoming power to the weld transformer according to a preset weld program as generated by the control module. The weld transformer will transform the incoming power to a high current pulse that is coupled to the contact tips to create a weld to a workpiece that is between the contact tips.
It has been long recognized in resistance weld applications that the load impedance of a resistance weld contains information regarding the quality of the weld and of the condition of the tooling attached to the weld control. In a lumped parameter model of the load impedance, the load impedance can be completely characterized by determining the maximum available weld current and the load circuit power factor. Some resistance weld controllers allow the user to specify an acceptable range of allowable circuit power factor and a range of maximum available weld current as an indicator of the condition of the weld process. In operation, the weld control estimates the load circuit power factor and maximum available weld current for a given weld and compares the estimated circuit power factor to the operator specified power factor range and the estimated maximum available weld current for the weld to the operator specified maximum available weld current range, and declares an event when one or both of these ranges is exceeded. An event as defined herein indicates the satisfaction of a logical condition that has been tested by the weld controller. An example of an event is when the measured power factor falls outside the operator specified range. The reaction of the weld controller to an event can be varied and may range from doing nothing that can be observed external to the weld controller, to illuminating a lamp indicating the event, or to aborting a weld sequence in progress.
The method of specifying ranges of power factor and maximum weld current is not very intuitive and places the burden of determining the power factor range and maximum current range on the operator, and requires the operator to understand the relation between a range in power factor or maximum current and a good weld. As a minimum, this requires experimentation on behalf of the operator to determine the appropriate range of maximum available weld current and power factor in order to create a balance between detecting potentially faulty welds and creating tripping. In a typical automotive application comprising between 2000 and 4000 individual resistance spot welds, this type of experimentation is highly impractical.
U.S. Pat. Nos. 5,386,096 and 5,449,877, henceforth the '096 and '877 patents respectively, describe methods for characterizing the load that do not depend upon the system computing the system power factor and maximum current. In the weld controls described in these patents, the weld control develops an internal model of the relation between weld current and the percent of maximum available heat as the weld control progresses through a ramp of heat or current defined by a stepper program which is a program that increases the programmed heat as the number of gun closings increase from a specified point in time. This is to compensate for the flattening of the tips of the weld gun as the weld gun opens and closes, which increases the contact surface area between the tips and the workpiece, decreasing the current density and therefore the temperature at the faying surfaces. Once the weld control has gathered enough data as defined by the operator, the operator is allowed to freeze the model of the relation so developed. The user then programs limits about the model that are either an offset from the model, a specific weld current above and below the data points of the model, or a proportional limit, which develops limits that are a percentage of the data points of the model. The systems of the '096 and '877 patents provide the distinct advantage of being more intuitive and understandable to the user because it uses parameters that the user can readily relate to the weld just produced rather than computing circuit model abstractions that are difficult to comprehend.
None of any of these approaches is capable of distinguishing between the short term variations that occur from workpiece to workpiece, and the longer term variations caused by incipient breakdown of the tooling, since the load impedance as measured by the weld control contains the lumped effects of both the workpiece and the tooling. It is normal for the impedance of the tool itself to change as the weld tool ages. Individual wires in cables connecting the transformer to the weld gun fatigue and break down. Shunts break and bolts become loose. Friction between moving parts of the tool can cause the pressure exerted on the workpiece to degrade. Additionally, the maximum available current is a function of the line voltage. In order to avoid nuisance indications that would become more frequent as the tooling breaks down, it is necessary in prior art systems to choose limits that accommodate both the long-term tool breakdown and variations due to the line voltage. This compromise made in order to minimize nuisance indications due to line voltage variations and long-term tool breakdown limits the sensitivity to which prior art weld controllers can provide indications of legitimate process problems. For tooling in a new configuration in which everything is operating as designed, a wider variation in workpiece impedance is required to exceed the programmed limits than would be the case when the tool is old and in need of service whereby the overall impedance trend of the tool has caused the expected lumped impedance of an ideal workpiece and the tool itself to drift toward one of the limits.
It would be desirable to have a weld control that can track long term tool degradation and distinguish between a short term problem in an individual workpiece just welded and long term variations due to tool degradation, which would allow a more sensitive indication of weld quality without generating nuisance indications. The present invention substantially achieves these objectives, while retaining the intuitive aspects of the prior art.